The uninsured rate has risen by 1.4 percent since the end of 2016, with the greatest increase being among those who paid for their own insurance either on or off the Affordable Care Act exchanges, according to a new Gallup and Sharecare poll.
The number of adults with self-paid plans has fallen from 21.3 percent to 20 percent.
Higher uninsured rates puts providers at risk for treating those without coverage who may also not be able to pay for care out-of-pocket.
The survey found the largest increases in the number of uninsured is from lower-income and middle-income adults rather than from higher-income consumers.
The 1.4 percent increase represents 3.5 million Americans.
The decline in self-paid insurance consumers represents a reversal of what had been the fastest growing type of coverage since the ACA's individual mandate took effect in the third quarter of 2013, when adults with insurance grew by 3.7 percent, the report said.
The uninsured rate peaked at 18 percent during the third quarter of 2013, prior to the implementation of the ACA.
Gallup and Sharecare said other ACA factors are contributing to the growth of the uninsured rate. These include higher premiums, fewer insurers offering ACA plans and uncertainty about the healthcare law as the GOP has several times tried to repeal it and President Donald Trump has implemented policies that have helped to destabilize the market.
Consumers may also feel that the individual mandate to have insurance or face financial penalties, will not be enforced.
"Without Congress and President Donald Trump taking steps to stabilize the insurance markets, the number of uninsured Americans likely will continue to rise," the report said. "Enrollment for 2018 plans on the ACA exchanges begins Nov. 1, and recent developments suggest premium costs will continue to increase."
The uninsured rate has risen by 1.8 points among those aged 35 to 64, compared with slightly smaller increases among younger adults.
The uninsured rate among black and Hispanic adults has increased by 1.5 and 1.6 points, respectively, since the end of 2016. During that same time period, the uninsured rate among non-Hispanic whites rose by 1.1 points.
The next largest change is among those with coverage under Medicare, which has fallen 0.5 points since 2016.
The percentage of U.S. adults lacking health insurance rose in the third quarter to 12.3 percent, up 0.6 percent from the previous quarter and 1.4 points since the end of 2016.
The uninsured rate is now the highest recorded since the last quarter of 2014 when it was 12.9 percent, the report said.
The uninsured rate had fallen to a record low of 10.9 percent in the third and fourth quarters of 2016.
The results for the third quarter of 2017 are based on more than 45,000 interviews with adults aged 18 and older, from July 1 to Sept. 30, conducted as part of the Gallup-Sharecare Well-Being Index.